The discovery on Wednesday was the first of a body of what may be water that has been found away from the polar ice caps and was revealed by the Mars Express spacecraft that has been orbiting and photographing the planet for a year.
Although the high resolution images only cover an area a few tens of kilometres across, they are in what appears to be a flood plain measuring a massive 800km by 900km.
The area is covered by a thick layer of volcanic ash, according to the European Space Agency (Esa) which has published the images on its website.
They show a flat plain that is covered with irregular block-like shapes.
"They look just like the rafts of fragmented sea ice that lie off the coast of Antarctica on Earth," the website said.
It said the water that formed the sea appears to have gushed up from beneath the surface through a series of fractures to form a catastrophic flood in the vast flood plain to an estimated depth of 45 metres.
Esa said as the water froze it broke into blocks which then became covered by an insulating layer of volcanic ash.
"Ice is unstable on the surface of Mars because of the low atmospheric pressure, and sublimates away (changes straight from ice to vapour without passing through the liquid state) into the atmosphere," it said.
"But some of the ice rafts appear to have been protected by layers of volcanic dust.
"While the entire sea froze solid, the unprotected ice between the rafts sublimated to leave 'ice plateaus' surrounded by bare rock," it added.
Esa said the relative absence of craters in the plateau suggested that the sea was a modern feature dating back no more than five million years.
"Two observations suggest that the ice is still there; first, the submerged craters are too shallow, indicating most of the ice is still in the craters; and second, the surface is too horizontal," it added.
Esa is planning to land a mission on Mars within the next five years.
Discovery of a large reservoir of water - along with oxygen, the space equivalent of gold - would be a major boost to manned deep space exploration.